1. Field of the Invention
The present invention relates to a transistor horizontal output driving circuit and more particulalry to a driving circuit suitable especially for a horizontal output circuit in a display unit using a cathode ray tube (CRT) that deals with a high horizontal deflection frequency, a power supply circuit and the like.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a horizontal deflecting circuit using a prior art horizontal output driving circuit. The circuit is comprised of a horizontal oscillating circuit 1 for outputting oscillating waveform Vosc synchronized to a horizontal synchronizing pulse P supplied from the preceding stage (not shown), the prior art horizontal output driving circuit 2, a horizontal output transistor 3, a damper diode 4, a flyback resonance capacitor 5, a horizontal deflection coil 6, an S-shape correcting capacitor 7 and a horizontal output transformer 8 (or a flyback transformer). The horizontal output driving circuit 2 is further comprised of a horizontal exciting transistor 9, a base input resistor 10 for the horizontal exciting transistor 9, a damping resistor 11, a damping capacitor 12, a horizontal exciting transformer 13, a current limiting resistor 14 and a base resistor 15 for the horizontal output transistor 3.
The horizontal deflecting circuit arranged as described above is often found in normal television receivers and the like and from its well-known principle, a half-sine horizontal flyback pulse Vc is generated at a collector of the horizontal output transistor 3 and sawtooth wave current Iy having horizontal deflection frequency synchronized with incoming synchronizing signal flows in the horizontal deflection coil 6. Because the horizontal deflection coil 6 is mounted on a neck portion (not shown) of the CRT, it can deflect electron beam in the CRT in the horizontal direction by the sawtooth wave current Iy.
When the circuit shown in the figure is used exclusively as a horizontal deflection circuit, the unit 8 works as a horizontal output transformer and a power is supplied to the circuit from a first DC power source +E.sub.B (DC voltage +E.sub.B) through a primary coil 8a of the transformer 8. When the circuit shown in the figure is used both as a horizontal deflecting circuit and a high voltage generating circuit, the unit 8 works as a flyback transformer, by which the horizontal flyback pulse Vc is boosted or stepped up at the side of a secondary coil 8b to produce a pulse Vhv. The pulse Vhv is then reflected to a high DC voltage to be supplied to the anode of the CRT to operate it.
Operations of the horizontal output driving circuit 2 and the horizontal output transistor 3 will then be explained.
When the horizontal oscillating waveform Vosc outputted from the horizontal oscillating circuit 1 is supplied to one end of the base resistor 10 through a terminal T, the horizontal exciting transistor 9 becomes ON i.e. conductive during this Vosc is in the high-level and exciting waveform Vdc is generated at its collector. The damping resistor 11 and the capacitor 12 are provided to prevent overshoot of the exciting waveform Vdc when the horizontal exciting transistor 9 is cut off. One end of a primary coil 13a of the horizontal exciting transformer 13 is connected to the DC power source +E.sub.B through the current limiting resistor 14.
A waveform having the same polarity with the exciting waveform Vcd is obtained in a secondary coil 13b of the horizontal exciting transformer 13. This waveform causes a base current Ib to the horizontal output transistor 3 through the base resistor 15. This base current Ib causes the horizontal output transistor 3 to perform ON-OFF switching operation and the switching of the horizontal output transistor 3 causes the circuit shown in FIG. 1 to work as a horizontal deflecting circuit as a whole.
FIG. 2 is an explanatory drawing of circuit operations of the horizontal deflecting circuit. When the output Vosc of the horizontal oscillating circuit 1 shown in FIG. 2A is supplied to the horizontal exciting transistor 9, the horizontal exciting transistor 9 becomes ON during the high-level period of Vosc plus storage time tsd which is determined by a characteristic inherent to the horizontal exciting transistor 9. As a result, waveform Vcd generated at the collector of the horizontal exciting transistor 9 becomes rectangular waveform as shown in FIG. 2B.
During a high-level period of the waveform Vcd, a voltage at one end of secondary coil 13b of the horizontal exciting transformer 13 connected to the base resistor 15, becomes positive. Then during the high-level period of the waveform Vcd, the base current Ib of the horizontal output transistor 3 flows in the positive direction as shown in FIG. 2C and turns the horizontal output transistor 3 to ON state making it conductive between its collector and emitter.
The base current Ib does not become zero-level suddenly even when the voltage waveform Vcd shown in FIG. 2B turns into bottoming state, but overshoots to go negative direction with respect to the zero-level, then goes positive direction to become zero when the storage time tso of the horizontal output transistor 3 ends, as shown in FIG. 2C. The ON state of the horizontal output transistor 3 continues until the storage time tso ends. Accordingly, collector current Ic of the horizontal output transistor 3 continues to flow until the end point of the storage time tso as shown by solid line in FIG. 2D.
When the storage time tso ends and the collector current Ic becomes zero, a half-sine flyback resonant pulse Vc as shown in FIG. 2E is generated at the collector. When this pulse Vc returns to zero-level, damper current Id automatically starts to flow as shown by dashed line in FIG. 2D and smoothly links with the collector current Ic. As a result, current Iy that flows in the horizontal deflection coil 6 becomes sawtooth wave in which the collector current Ic, the damper current Id and a current (not shown) that flows in the flyback resonant capacitor 5 are combined (see FIG. 2F). This sawtooth wave Iy deflects the electron beam of the CRT in the horizontal direction.
The horizontal deflecting circuit arranged as shown in FIG. 1 performs satisfactory provided that the horizontal deflection frequency is low e.g. 15.73 kHz for normal TV broadcasting receivers.
However in image display units used lately for CAD and the like, horizontal deflection frequency becomes increased to such as 84 kHz, 97 kHz and 128 kHz as a design trend to meet the demand for increasing definition of images. When such a horizontal deflection frequency is increased (when the horizontal deflection period th is shortened) like that, value of the storage time tso aforementioned becomes critical. For example, when a horizontal deflection frequency is 100 kHz, i.e. a horizontal deflection period th is about 10 .mu.s, because the storage time tso of a horizontal output transistor is generally about 4 to 5 .mu.s, the storage time tso becomes about half of the horizontal deflection period th and the operations as shown in FIG. 2 cannot be performed. Therefore, in order to deal with such a high horizontal deflection frequency, the storage time tso has to be shortened as small as possible.
In order to reduce the storage time tso, one method is known, in which an inverse base current Ibn shown in FIG. 2C is increased to sweep excessive carriers in the base layer of the transistor 3. That is, as shown in FIG. 3, when the absolute value of the inverse base current is increased from Ibn1 to Ibn2, the storage time tso is shortened from tso1 to tso2.
However, it is difficult to increase the value of the inverse base current alone in the prior art shown in FIG. 1. For example, the inverse base current Ibn may be increased when the winding ratio of the horizontal exciting transformer 13 is increased, but base current Ibp in the forward direction also increases in the same time. If the base current Ibp in the forward direction increases, supersaturation state develops in proportion and causes the storage time tso to be prolonged.
An existence of a leakage inductance of the secondary coil 13b of the horizontal exciting transformer 13 is also a problem. The leakage inductance reduces the waveform inclination of the inverse current Ibn and prolongs the time for sweeping all the excessive carriers, i.e. the storage time tso.
Then, a means for forcibly inducing the inverse base current Ibn, is required regardless of the design of the horizontal exciting transformer 13 in a horizontal deflecting circuit used in a high definition display unit that deals with high horizontal deflection frequencies.
Fig. 4 is a circuit diagram showing a main part of horizontal output driving circuit in which a means for forcibly inducing inverse base current Ibn is provided. This is disclosed in the Japanese Laid-open Patent Application No. 61-20474/1986 which the present applicant applied. In this circuit, a second base resistor 16, an Ibn inducing circuit 17, a timing circuit 18 and a second DC power source -E are newly added. The Ibn inducing circuit 17 is a type of gate circuit which becomes conductive during the period of the aforementioned storage time tso and causes the inverse base current Ibn flowing to the DC power source -E through the resistor 16. The timing circuit 18 outputs a rectangular wave Vdn having horizontal deflection period and determines a timing of gate operation of the Ibn inducing circuit 17.
Such circuit arrangement allows to flow an enough inverse base current Ibn with the negative power source -E and to shorten the storage time tso.
However, the circuit shown in FIG. 4 had a problem that the circuit configuration becomes complex and larger in proportion to the means provided for forcibly inducing the inverse base current Ibn.
Moreover, in both circuits shown in FIGS. 1 and 4, the horizontal exciting transformer 13 within the horizontal output driving circuit 2 performing the switching operation with the high frequency rectangular wave, causes a significant amount of power loss in its core unless an expensive core material having excellent high frequency characteristics is used.
Furthermore, a current is abruptly interrupted in the primary coil 13a of the horizontal exciting transformer 13, so that transient develops in the primary side voltage waveform Vcd. If a peak value of the transient exceeds the maximum allowable voltage of the collector of the horizontal exciting transistor 9, it will damage the horizontal exciting transistor 9. Therefore, a damping circuit comprised of the resistor 11 and the capacitor 12 is required. However, there has been a problem associated with the provision of the damping circuit which introduces a loss at the resistor 11 and a slow rise of the voltage waveform Vcd. The latter increases a collector loss of the horizontal exciting transistor 9, accordingly the efficiency of the circuit as a whole is lowered.
It is, therefore, an object of the present invention to provide a horizontal output driving circuit that allows to shorten the storage time tso of the transistor serving as a switching device so that its switching speed is improved, by a simple circuit arrangement without sacrificing the efficiency of the horizontal output circuit.
Another object of the present invention is to provide a horizontal output driving circuit that is suitable for a horizontal deflecting circuit that deals with a high horizontal deflection frequency and for a power circuit that performs a high speed switching.